The genetic and cytogenetic basis of various diseases with mental retardation will be determined through an integrated application of such tools as tissue culture, hybrid cell techniques, enzymatic analysis, and cytogenetic studies. Emphasis will be placed upon developing an understanding of mammalian regulatory genes, assigning genes to specific chromosomes, methods of introducing exogenous DNA into cultured mammalian cells, and studies of chromosome structure. The results obtained will be used to elucidate the etiology and pathogenesis of various diseases associated with mental retardation and to develop new therapeutic approaches. BIBLIOGRAPHIC REFERENCES: Davidson, R.L.: "Control of Differentiated Functions in Somatic Cell Hybrids," Ontogenez (USSR), 1975. Kaufman, E. and Davidson, R.L.: Control of the Expression of a Herpes Simplex Virus Thymidine Kinase Gene Incorporated into Thymidine Kinase-Deficient Mouse Cells. Somatic Cell Genetics 1:151-161, 1975.